Internet of Things, IoT, is expected to increase the number of connected wireless devices significantly. A number of devices, e.g. household appliances such as microwave ovens, operate at frequencies about 2.4 GHz. The electromagnetic emissions from these devices risk producing interference with wireless communication devices operating near the same frequency. To avoid interference from devices not intended for wireless communication certain frequency bands have been reserved for wireless communication purposes via international agreements. The use of the reserved frequency bands are regulated using licenses, which is why these bands are often called licensed bands. Analogously, bands not reserved and hence not regulated using licenses are called unlicensed bands. The 2.4 GHz band which is mainly intended to be used for industrial, scientific and medical applications, ISM, is an example of an unlicensed band.
A vast majority of the IoT-devices will likely operate in unlicensed bands, in particular the 2.4 GHz ISM band. At the same time, there is also increased demand for using the unlicensed bands for services that traditionally have been supported in licensed bands. As an example of the latter, 3GPP, that traditionally develop specifications only for licensed bands has now also developed versions of Long Term Evolution, LTE, which will operate in the 5 GHz unlicensed band.
Technologies that are expected to dominate for IoT services are Bluetooth Wireless Technology, in particular Bluetooth Low Energy, BLE, and future versions of IEEE 802.11 like 802.11ax. With respect to IEEE 802.11, there are currently efforts to standardize an integrated long range low power, LRLP, mode which at least to some extent builds upon the above mentioned 802.11ax.
IoT applications are foreseen to most often have rather different requirement and features compared to applications like e.g. file down-load and video streaming. Specifically, IoT applications would typically only require low data rate and the amount of data transmitted in a single packet may many times only be a few bytes. In addition, the transmissions to and from many devices will be very seldom, e.g. once an hour or even less often. However, the number of IoT devices is expected to be huge, which means that although the amount of data to each one of the devices may be small, the aggregated IoT data may still be substantial. Many use cases for IoT applications can be found in an ordinary home, and may be related to various sensors, actuators, etc. The requirements for coverage are therefore substantially less demanding than what usually can be achieved by e.g. a cellular system. On the other hand, the coverage which can be obtained by e.g. Bluetooth or 802.11b/g/n/ac may not suffice. This may be in particular true if one of the devices is outdoors whereas the other device is indoors so that an exterior wall with rather high penetration loss is in between the devices. Due to this shortcoming of current versions of Bluetooth Wireless Technology and IEEE 802.11, both these standardization organizations are working on new versions that would significantly increase the coverage.
The straightforward approach to increase the range of a communication link is to reduce the bit rate that is used. Reducing the bit rate by necessity means that it will take longer to transmit a packet of a certain size. As a side effect of this, the channel will be occupied for a longer time. Now, with a large number of devices sharing the same channel, the channel may be congested if this sharing is not done in an effective way. The need for long packets and the increased number of users will make this congestion even more pronounced.
Moreover, the amount of non-IoT data, e.g. data down-load and video streaming, transmitted over the same channel may also increase. This implies that to obtain good performance for both IoT applications and non-IoT applications, some coordination should preferably take place. Today there is no single standard that effectively supports both high-data rate application and really low cost IoT applications, like sensors. The main standard for the former is IEEE 802.11, e.g. 802.11n and 802.11ac, whereas the main standard for the latter is Bluetooth Low Energy. Hence, typically two systems need to operate in parallel and preferably in a synchronized fashion.
An obvious, and probably the simplest, way to do such coordination is by time sharing between the systems. For example, each system is assigned time slots where data may be transmitted or received according to a predetermined scheme. This is commonly referred to as Time Division Multiplexing, TDM. In each time slot assigned to a specific system, this system may then for instance use Time Division Duplex, TDD, which is a common way of implementing time sharing, wherein users are assigned time slots for uplink and downlink transmission. The main reason for TDD is that it allows for low cost implementation without the need for costly duplex filters, which are needed in case frequency division duplex, FDD, is employed. However, as the data rate for the IoT system is very low for the individual links, it may likely be hard to obtain good spectrum efficiency in this way.
Instead it would be preferable if the two systems, i.e., both the IoT system and the non-IoT system could operate concurrently. One means to achieve this could be if the non-IoT system would be based on orthogonal frequency division multiplexing, OFDM. Concurrent operation could then be achieved by assigning one or more subcarriers to the IoT system and the remaining ones to the non-IoT system. The amount of subcarriers allocated to the IoT system could in this way be rather flexible.
The approach of using OFDM is conceptually simple and is also the approach suggested for the Long Range Low Power mode currently discussed within IEEE 802.11. Although this clearly is an attractive property, it does not address the even more important question namely how to build extremely low cost and low power devices.
Hence, there is a need for network nodes that support concurrent operation with different types of wireless devices, one type able to transmit and receive high data rates such as OFDM, the other only able to transmit and receive considerably lower data rates.